Inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a inkjet firing chamber resistor, located opposite the nozzle so ink can collect between it and the nozzle. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Color inkjet printers commonly employ a plurality of print cartridges, usually either two or four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge contains a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge usually contains black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks. The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
When a number of pixels in a particular area of an absorbent print medium such as bond paper absorb the liquid solvent constituent (typically water) of the ink, the paper fibers in that area will expand until the solvent has evaporated or otherwise dispersed. Because the dampened area of the print medium is typically constrained in the plane of the paper by adjacent less damp areas and/or by the paper advance mechanism and from below by a platen, the dampened area has a tendency to buckle upwards towards the nozzle (a problem referred to as "cockle"). If the height of the buckle exceeds the nominal spacing between the pen and the paper, then the ink in that area will be scraped by the pen as the pen retraces over some or all of the buckled area during a subsequent sweep over the same in the opposite direction (bidirectional and certain color printing modes) or prior to printing a sweep over an overlapping area (multiple pass printing modes). Such scraping causes smearing of the still damp ink and a degradation of image quality.
A related problem is "curling" of the paper. As a result of the differential absorption of solvent on the two sides of the paper, once the paper exits from the feed mechanism, it is no longer under tension and has a tendency to curl. Depending upon the extent of the curl, which is a function of both overall image density and throughput speed, the printed surface will be urged against various stationary parts of the printer between the carriage and the output tray, and at least the densest parts of the image will be smeared.
The print medium becomes damper and remains damp for a longer time as more ink is applied on the same area of the print medium. Thus, the probability of buckling or curling increases when ink density of a print image increases to produce intense black or colored portions of the image. The probability of smearing also increases when the speed of the printer increases and less time is allowed for the ink to dry, or when the distance between the paper and the nozzle is reduced to more accurately define the size and location of the individual dots of ink. Problems associated with scraping of the nozzles against the raised portions of the image are most noticeable during high quality multiple pass printing modes in which the nozzle passes several times over the same area. The curling problem is particularly noticeable in high quality, high throughput (single pass) printing modes in which a large quantity of ink is deposited over a relatively large area in a relatively short time.
Prior printers were designed so that each printhead was the same distance from the media. The distance was determined by adding up the various tolerances such as media cockle height, tolerance between the parts that define the position of the media and the carriage, tolerance from printhead location to printhead location within the carriage, and variation in the distance from the closest part of the printhead to the media to the surface on the print cartridge that locates the printhead in the carriage. These tolerances can require a nominal printhead to media distance that does not produce good print quality due to the increased effects of spray and errors in the nominal trajectory of the main drop.
Moreover, this does not yield optimum print quality for a black and color printer, since the nominal printhead to media distance is identical for the black and color pens. Black text print quality is more sensitive to printhead to media spacing than color graphics and images are, therefore having the black printhead further from the media than the color printhead(s) and all the printheads far enough from the media to prevent scraping will produce a lower print quality than could be achieved if it was possible for the black printhead to be closer to the print media.
One known solution of the scraping problem is to increase the spacing between the pen and the print medium. However, because such an increase in spacing would reduce the precision and sharpness of the ink drops and thus degrade the print quality, that solution is not satisfactory for printing high quality graphics applications.
Another known solution of the smearing problem is to accelerate the evaporating of the solvent by heating the print medium as it is being printed and/or circulating dry air over the freshly printed image; however excessive heating interferes with the proper adherence between the ink and the print medium, and may also cause the less densely inked areas to shrink and/or to become brittle and discolored. These problems may also be avoided by providing a relatively long fixed time delay between successive sweeps by the pen. However, such a solution would decrease the throughput of the printer. At a time when the industry is in a pursuit to increase the throughput of printers so that they can keep up with the increasing throughput of central processing units, such a solution is unsatisfactory.
Thus, the prior art has failed to provide a satisfactory solution for printing a high quality graphics image at a high throughput rate, which is further exacerbated if additional dots of ink are selectively applied between adjacent pixels, thereby effectively doubling the number of dots of ink, in order to increase image density and/or to provide a smoother boundaries for any curved or diagonal images ("Resolution Enhancement Technology").